Catalytic convertors are well known for the removal and/or conversion of the harmful components of exhaust gases. While catalytic convertors have a variety of constructions for this purpose, one form of construction is a catalytically coated rigid skeletal monolithic substrate, or honeycomb-type element which has a multiplicity of longitudinal channels to provide a catalytically coated body having a high surface area.
The rigid, monolithic substrate is fabricated from ceramics and other materials. Such materials and their construction are described, for example, in U.S. Pat. Nos. 3,331,787 and 3,565,830 each of which is incorporated herein by reference.
The monolithic substrate and particularly the multiplicity of channels are coated with a slurry of a catalytic and/or absorbent material. While various methods are known in the art for coating a monolithic substrate with a catalytic slurry, such methods from the standpoint of cost are deficient in minimizing the amount of coating applied, especially when a costly catalytically active precious metal such as platinum, palladium or rhodium is deposited as part of the coating. Not only is it difficult to coat monolithic substrates, it is also difficult to provide a consistent and reproducible coating pattern within the channels.
One method of coating a prefabricated monolithic substrate is to pump the catalyst slurry into the respective channels and then subject the coated substrate to a drying operation. Such systems have been unsuccessful in providing a uniform coating thickness and a uniform coating profile wherein the catalyst coating is deposited over the same length of each of the channels.
It has been proposed to employ a vacuum to draw the catalyst slurry upwardly through the channels. For example, Peter D. Young, U.S. Pat. No. 4,384,014 discloses the creation of a vacuum over the monolithic substrate to remove air from the channels and then drawing the catalyst slurry upwardly through the channels. The vacuum is then broken and excess slurry is removed, preferably by gravity drainage.
James R. Reed et al., U.S. Pat. No. 4,191,126 disclose the dipping of the monolithic substrate into a slurry and then utilizing subatmospheric pressure to purge the excess coating slurry from the surfaces of the support. The applied vacuum is intended to unplug the channels so that the slurry is drawn over the surfaces of each of the channels.
An improvement in these systems is disclosed in Thomas Shimrock et al., U.S. Pat. No. 4,609,563, incorporated herein by reference. This system encompasses a method of vacuum coating ceramic substrate members with a slurry of refractory and/or catalyst metal components wherein precisely controlled, predetermined amounts of the slurry are metered for application to the ceramic monolithic substrate. The monolithic substrate is lowered into a vessel of preferably predetermined dimensions to a predetermined depth containing the precise amount of slurry which is to be coated onto the substrate. The slurry is then drawn up by a vacuum which is applied to the end of the substrate opposite to the end which is immersed in the bath. No draining or purging of excess coating slurry from the substrate is necessary nor is any pre-vacuum application step required to eliminate air.
The design of the vessel also known as a dip pan which contains the precise amount of slurry in U.S. Pat. No. 4,609,563 is desirably shaped to freely receive but closely conform to the shape of the substrate to be coated. Thus, if the monolithic substrate is in the shape of an oval, the dip pan is in the shape of an oval with slightly larger dimensions than the substrate itself.
While the '563 patent process provides a smooth coating exceeding that of the other reference processes, nonetheless, there is still difficulty in obtaining a uniform coating profile such that the coating covers the same length of each channel. In addition, because the '563 patent process prefers precisely shaped and dimensioned dip pans for each type of monolithic substrate, added expense is incurred by having to inventory a variety of different sized and shaped dip pans. Still further the dimensions of each dip pan is preferably only slightly larger than the substrate. Therefore extra care must be taken to place the fragile substrate in the dip pan so that the substrate does not accidently hit the dip pan.
It would therefore be a significant benefit in the art of coating monolithic substrates and particularly monolithic substrates for use in catalytic convertors if each channel can be coated with the same thickness of coating for the same length so as to provide a uniform coating profile in a cost effective and efficient manner.